The present invention relates to the jacketing of metallic shapes and sections such as steel pipes by means of thermoplastic material, particularly polyethylene.
German printed patent application No. 12 61 431 discloses the coating of metal surfaces by means of a polyethylene jacket in that initially a blend of an epoxy resin and of a hardening and curing agent is deposited upon the metal surface and curing is permitted to begin, whereafter, by means of a flame-spray gun, some polyethylene is sprayed upon the coated metal surface, and after curing of the epoxy resin layer has been completed, additional polyethylene is sprayed upon, whereby the temperature of the metal surface is not to exceed 100-degree centrigrade. In accordance with the particular example set forth in that publication, curing of the epoxy resin layer occurs at a temperature not exceeding 60 degrees Centigrade and for about 24 hours. For mass production of jacketed steel pipes, this method is uneconomical because a large throughput is incompatible with such a long curing period for the epoxy layer.
In accordance with the German printed patent No. 19 65 802, the jacketing of steel pipes by means of polyethylene has been proposed, in which the pipes, at first, are heated to a working temperature above 100 degrees Centrigrade, whereupon a base layer is coated upon the heated tube which will crosslink at that temperature; the layer is preferably an epoxy resin. Subsequently to the curing of the epoxy resin and particularly immediately after the completion of the curing process, a thin ribbon of ethylene copolymer material is extruded and wrapped around the epoxy resin coated tube and a polyethylene ribbon is also extruded and wrapped around the ethylene copolymer ribbon, the latter serving as an adhesive for the polyethylene ribbon. The throughput and operational speed of this procedure is limited because a certain period of time is needed to permit evaporation and volitalization of the reaction products of the curing and crosslinking processes of the epoxy layer. The method, moreover, is therefore limited to the utilization of coating and jacketing tubes having a diameter in excess of 600 mm.
In accordance with the German Pat. No. 22 22 911, it is known to provide the following procedure. A steel pipe, having a temperature in the range of 70 degrees to 90 degrees Centigrade and having been coated in its interior with a heat-sensitive layer, is provided with a base layer made of a curable expoxy resin blend, upon which a coating is extruded as a kind of twin-hose consisting of an ethylene copolymer and an outer polyethylene layer. After the jacketing has been completed, the tubes are cooled at room temperature at a dwell time of about 3 minutes. Complete curing requires a longer time and, in fact, curing has been completed at room temperature and 65% ambiant air humidity only after an elapse of about 24 hours. This relatively long period of completion of the curing process is again detrimental to the overall throughput. Moreover, the initial process parameters have to be maintained very carefully. In the case of deviation, further delay of the curing is incurred or a completion of curing may be prevented entirely.
In order to improve the aforementioned method, the German Pat. No. 22 57 135 proposes to increase the surface temperature of the steel pipe to be jacketed to a temperature of about 80 degrees Centrigrade and to provide an electrostatic coating by means of a solution-containing epoxy resin and a curing agent blend, the layer thickness is about a 100 micrometers. This layer serves as a base upon which the thermoplastic jacket is provided. This thermoplastic layer may consist of an inner ethylene copolymer and an outer polyethylene layer. The thus jacketed tubes or pipes are then cooled in water to a mean temperature of the tube of about 40 degrees Centigrade. Subsequently, the core tube is inductively heated to a mean temperature of about 100 degrees Centigrade. The surface temperature of the tube will, in fact, rise to about 240 degrees Centigrade. At this temperature, curing of the epoxy resin base layer occurs within a few seconds. Subsequently, the jacketed tube is cooled back to room temperature.
It has been found, however, that in many cases of practicing this last-mentioned method, the depositing of a twin-hose coating by means of extrusion upon tubes or pipes which are between 6 meters or 12 meters long, difficulties occur in that the evaporation of the solvent from the epoxy-resin curing agent blend is insufficient for a pipe speed of, say, 20 meters per minute, as envisioned in this method. This is compounded by the desired layer thickness and the existing temperature of the tube prior to depositing the twin-tube. On the other hand, a method of jacketing a tube having a diameter larger than 600 meters by means of a strip-shaped twin-foil is quite suitable.